1. Field of the Invention
The invention relates generally to the in vitro prediction of a composition's sunscreen PFA values. More particularly, the present invention relates to an in vitro method for the prediction of in vivo UVA protection by a composition with sunscreen properties.
2. Description of Related Art
It is known that the UV portion of sunlight causes skin damage. UVB irradiation is almost entirely absorbed by the epidermis and it causes an erythemal or sunburn reaction, as well as induces DNA mutations.
UVA irradiation is capable of reaching dermal layers and even affecting circulating blood cells. Compared with UVB, UVA generates more oxidative stress, which induces matrix metalloproteases and suppresses skin immune function.
Sunscreen compositions or any compositions with sunscreen active material are designed to protect skin from photodamage that may result from UV light exposure. It is becoming more important to develop sunscreens that effectively protect from both UVB and UVA.
UVB protection of a sunscreen is measured by a FDA approved SPF in vivo method that utilizes erythema as a biological endpoint. In contrast to the SPF method there is no official method to measure photoprotection against UVA. At the same time, effective protection against the UVA portion of the solar spectrum associated with cumulative skin damage is an important element of sunscreen and anti-aging cosmetic formulations.
The JCIA (Japan Cosmetic Industry Association) method is an in vivo method that is the most often used in the U.S. According to this method, Protection Factor A (PFA) based on persistent pigment darkening (PPD) can determine UVA protection provided by sunscreens. It was adopted by JCIA in 1995 as an official method. Additional methods modeled after the JCIA method are described in The Reproducibility of an In-Vitro Determination of the UVA INDEX Describing the Relative UVA Protection of Sun Care Products, Gers-Barlag et al., IFSCC Magazine, Vol. 5, No.3, 2002; and A New In-Vitro Test Method to Assess the UVA Protection Performance of Sun Care Products, V. Wendel et al., SOFW 2001, 127.
Since PPD is produced in the basal keratinocytes by a photochemical conversion of preexisting melanin and its precursors and/or migration of melanosomes, it may be assumed that PPD gives a direct estimate of the UVA impact on the viable epidermis. PPD is a stable skin response that is linearly dependent on the amount of UVA that enters the viable epidermis. In the JCIA method, the PFA is determined from the ratio of the sunscreen-protected minimal PPD to the unprotected PPD, evaluated 2 to 4 hours after UVA exposure. This method utilizes a xenon arc light simulator filtered with a 2 millimeter (mm) WG335 and 1 mm UG11 filters and volunteers with skin types II, III and IV.
Time of UVA exposure during in vivo PFA testing of a sunscreen is based, according to the test protocol, on its PFA estimated value that was determined in vitro prior to the human tests. Thus, correct in vitro prediction of the PFA range of the test product is absolutely essential for the accuracy of in vivo tests and must be given to the testing lab prior to the study on volunteers. In the prior art methods, in vitro UVA protection is calculated using the integration area of normalized spectrum of a sunscreen in entire UVA region (UVA1+UVA2), which does not result in the accurate prediction of in vitro PFA values.
To overcome the deficiencies and inaccuracies associated with the prior art methods, the present invention provides a method for accurately predicting the in vitro PFA range for suncare compositions.